Simple onrollout function

Hi, I’m new to all this flash business and through some basic tutorials I have managed to turn a movie clip into a button, so that when the mouse is released the clip plays. My problem is that when the mouse rolls out the clip returns to the beginning state. This beginner would appreciate some help!

so far the actionscript I have

this.button_mc.onRollOver = function(){
button_mc.gotoAndPlay("_over");
}

this.button_mc.onRelease = function(){
button_mc.gotoAndPlay("_down");
}

thanks
t